1. Field of the Invention
The present invention relates to a method for producing regenerated cellulose fibers having excellent flameproofness.
2. Description of the Prior Art
Many methods have been proposed which impart flameproofness to regenerated cellulose fibers. The latest, well-known, commercial method is disclosed in U.S. Pat. No. 3,371,131 where a polyphosphonate which is represented by formula (2) and obtained by polymerizing a cyclic chlorophosphite and a ketone compound in an acidic medium ##STR2## WHEREIN D is an alkylene having 2 or 3 carbon atoms or an alkyl substituted alkylene in which the alkyl substituent has a total of not more than 8 carbon atoms and the alkylene has 2 or 3 carbon atoms; R.sub.1 and R.sub.2 are alkyl, phenyl, phenylalkyl, alkylphenyl, alkenyl or alkoxycarbonyl and R.sub.1 and R.sub.2 contain no more than 12 carbon atoms, and n is an integer of 1 to 1000, is blended with a viscose to be spun. However, this method presents several problems. One of these problems is that the polyphosphonate represented by formula (2) is partially hydrolyzed with alkali in the viscose and has a tendency to shed into the coagulation bath in the spinning step which causes a substantial reduction in the amount of the polyphosphonate retained in the fiber. Another problem is that because the polyphosphonates hydrolyze during preparation of the viscose fibers, the waste water from the process contains low molecular weight phosphorous compounds and therefore presents a pollution problem. All of these troubles result from the characteristics of the polyphosphonates represented by formula (2).
That is, the phosphorus-containing polymers disclosed in U.S. Pat. No. 3,371,131 have an extremely broad molecular weight distribution for the reasons that no polymerization regulator (reaction-terminator) is used and that the polymerization velocity is relatively fast because the reaction is conducted in an acidic medium. For example, even those polyphosphonates having a mean molecular weight of 7000 include some extremely low molecular compounds. Furthermore, a cyclic phosphite structure or an active chloride remains at the molecular terminal of the phosphorus-containing polymers. Of course, conversion of trivalent phosphorous to pentavalent phosphorous occurs partially by the heat treatment during synthesis, but in any case, it is certain that the polymer terminals are in a very unstable state. The ability of such polyphosphonates to hydrolyze when treated with alkali bears a close relationship with the terminal structure of the polymer. Further, with a decrease in the molecular weight, which represents an increase in the number of terminals, or with an increase in the degree of instabiity of the polymer terminals, the polyphosphonates are more easily decomposed and so the retention of the polyphosphonate in the fibers is reduced. Still further, difficulties are apt to occur in the yarn spinning step. Therefore, in order to solve these troubles, it is important (1) to fix the terminals of the polymer and to adjust the degree of polymerization with a polymerization regulator and (2) to find the conditions under which good polymers of narrow molecular weight distribution (distribution of polymerization degree) are obtained by reducing the polymerization velocity as much as possible.
In view of the considerations above, detailed experimentation has led to the successful synthesis of polyphosphonate compounds having excellent hydrolysis resistance which is disclosed in a United States patent application filed concurrently herewith which is based on priority to Japanese patent application 46549/1975, filed Apr. 18, 1975.